The present invention relates to cellular cross-flow bodies, as well as an extrusion die and method for making such bodies. In particular, the invention relates to a die and method of extruding cross-flow honeycomb structures useful for various applications such as heat exchange, filtration, catalysis, oxygen production, and energy production. In cross-flow applications, gases or fluids flow in more than one direction through the structure.
Honeycomb monolithic structures are generally composed of straight flow-through cells. However, such flow-through structures are not appropriate for certain applications such as where it is desirable to have the fluid make several passes through the channels before it is discharged, where cross-flow channels are desired such as in heat exchangers, or when increased turbulence would be beneficial. Multiple passes and prolonged contact lead to more thorough heating and/or cleaning as the fluid is allowed prolonged contact with the heat exchanger, catalyst or filter.
Traditional methods for making structures with non-parallel channels or passages generally require multiple steps. For example, in one approach a cellular ceramic body is cut and plugged so as to form non-parallel flow directions. In another approach often used for cross-directional flow structures such as heat exchangers and fuel cells, layers of green or fired sub-assemblies are formed by frit-bonding. This is the method often used for fuel cells where monolithic and planar structures contain non-parallel channels for fuel and air such as found in heat exchangers. Typical cross-flow structures (e.g., heat exchangers), are formed by first extruding a honeycomb-like body of ceramic material from a die orifice. This extrusion results in a block of ceramic material having straight-flow channels or cells which are generally of square or other rectangular cross-section, arranged parallel and adjacent to one another along the axis of extrusion. To form a cross-flow structure, portions of the sides of the extruded ceramic block are cut away to convert the ceramic block having straight-through passages into a composite block having alternating rows of straight-through flow, and Z-flow, L-flow, U-flow or other similar cross directional flow through the ceramic block. The cross-flow (Z-flow, L-flow, etc.) channels are then made by sawing into the sides of some of the channels in the ceramic block and afterwards sealing the ends of these channels, thereby forming the cross-flow channels.
Various methods have been suggested for making cross-flow structures for example, by sawing and stacking. In general, the suggested methods have required multiple steps. For example, in one approach a cellular ceramic body is cut and plugged so as to form non-parallel flow directions. In another approach, green or fired sub-assemblies of ceramic material are stacked and bonded together by sintering or frit-bonding. It has also been suggested to use the sawing technique to produce an L-flow cross-flow heat exchanger in which both flow directions through the heat exchanger follow an L-shaped path. When such sawing techniques are utilized to make cross-flow heat exchangers, very high precision extrusion geometries are required, as well as high precision cutting equipment, to arrive at a good quality finished cross-flow heat exchanger. Imprecision in either the extrusion or the cutting equipment can result in leakage paths between channels, which has a deleterious effect on heat exchanger performance. Further, because such heat exchangers are typically made by sawing into the side of the extruded ceramic body, it is very difficult to consistently achieve precise uniform cutting of the ends of the ceramic body. Such inconsistencies can result in undesirable leakage paths between adjacent channels.
Cross-flow heat exchangers having straight through flow channels in two directions have been disclosed in which layers having upstanding ribs thereon are laid one on top of another to form a heat exchanger having alternating layers of straight through flow channels, every other layer being arranged in a transverse direction to the one before it. The upstanding ribs of these layers in the green state are relatively weak, due in large part to their relative lack of support. Consequently, these methods sometimes result in the ribs being bent either prior to or during the stacking process. Furthermore, because each directional flow layer consists only of one layer of channels, the manufacturing process is relatively time consuming and labor intensive. To date, traditional extrusion dies have proved inappropriate for producing cellular structures of the type described above where the channels are not necessarily parallel. Such methods have proved both difficult and expensive for non-parallel, cross-flow dies due to the many processing steps often required to produce useful dies.
To overcome some of the above problems, recently in co-pending, co-assigned U.S. Ser. No. 08/132,923 (Faber et al.), a method has been suggested for forming self-supporting cellular structures by extruding relatively soft batches into a drying medium or by contacting the formed structure with a drying liquid immediately as the structure exits the extrusion die.
More recently, in co-pending, co-assigned U.S. Ser. No. 08/341667 (St. Julien), a moveable die is disclosed which is capable of producing honeycomb structures some of whose cells may be perpendicular to the axis of extrusion, and which structures may be used as cross-flow bodies.
There continues to be a need for easier, more effective and less expensive methods for making cross-directional flow structures and other cellular structures in which the cells are not always parallel to the axis of extrusion. Accordingly, the object of the present invention is to provide an extrusion die and method of making geometrically complex cell directions such as cross-flow structures in which the cell directions are not always parallel to the axis of extrusion.